Where SCR catalysts are employed to limit NO.sub.x emissions from diesel engines, one has to deal with either the dangers of ammonia or risk fouling the catalysts under most conditions. In this regard, see R. J. Hulterman; A Selective Catalytic Reduction Of NO.sub.x from Diesel Engines Using Injection Of Urea; Ph.D. thesis, September 1995. Hulterman describes a number of technical challenges including clogging of atomizers, decomposition problems and system dynamics. Similarly, in SAE Paper No. 970185, entitled "Transient Performance of a Urea DeNOx Catalyst for Low Emissions Heavy-Duty Diesel Engines", it is indicated that the injection nozzles must protected from undue heat.
The limited attempts to use urea SCR for diesel engines have required the use of large pyrolization chambers or other devices following the point of urea introduction into the exhaust, as disclosed in U.S. Pat. No. 5,431,893, to Hug, et al. Equipment of this type highlights the known problems with urea. Once introduced into diesel exhaust, urea takes time to break down and may cause nozzle plugging as conventionally introduced and still as proposed in that disclosure. To protect the catalyst from fouling, Hug, et al., propose bulky equipment. In addition, this disclosure highlights the necessity of maintaining the urea solution at a temperature below 100.degree. C. to prevent decomposition prior to passage through the nozzle. They propose the use of moderate urea pressures when feeding the urea and find it necessary to have alternative means to introduce high-pressure air into the feed line when it becomes plugged. The nozzles employed by Hug, et al., are apparently capable of producing moderately-fine sprays, the dispersion of which is aided by auxiliary air, but the droplets are still large enough to require a large pyrolization channel. See also, WO 97/01387 and European Patent Application 487,886 A1.
Each of the points of caution by these references about the difficulties of using urea with SCR systems, especially for mobile sources, points to the trouble the art has had and continues to have.
The art is awaiting the development of a process and apparatus that would permit the use of urea in an SCR process simply, reliably, economically and safely.